1. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a method and an apparatus for allocating an Acknowledgement/Negative Acknowledgement (ACK/NACK) channel.
2. Description of the Related Art
Long Term Evolution (LTE) systems of the related art have a maximum bandwidth of 20 MHz which cannot meet the requirements for higher data transmission rates. LTE-Advanced (LTE-A) is brought forward based on LTE to increase transmission rates of users. In LTE-A systems, multiple Component Carriers (CC) are aggregated to generate a larger working bandwidth for both downlink and uplink of the communication systems, thus a higher transmission rate can be provided. This technique is referred to as Carrier Aggregation (CA). For example, 5 CCs of 20 MHz may be aggregated to provide a bandwidth of 100 MHz. Each CC is referred to as a cell.
Among multiple downlink Cells configured in a Base Station (BS), one Cell is a Primary cell (Pcell), and other Cells are referred to as a Secondary cells (Scell).
The BS may configure a User Equipment (UE) to receive downlink data from multiple Cells by utilizing high layer signaling, but the actual number of Cells dynamically scheduled within a sub-frame may be smaller than or equal to that of the Cells configured by the high layer signaling. Herein examples are described below with reference to FIGS. 1 and 2.
FIG. 1 is a schematic diagram illustrating a BS scheduling a Cell according to the related art.
Referring to FIG. 1, four Cells are configured via high layer signaling, denoted as Cell 1 to Cell 4. However, the BS schedules only one Cell, i.e., Cell 1.
FIG. 2 is a schematic diagram illustrating a BS scheduling three Cells according to the related art.
Referring to FIG. 2, four Cells are configured via high layer signaling, denoted as Cell 1 to Cell 4. However, the BS schedules only three of them, i.e., Cell 1, Cell 2 and Cell 3.
Data transmission in a downlink Cell may be scheduled using Physical Downlink Control Channels (PDCCHs) transmitted in other Cells, and this manner is referred to as cross carrier scheduling. Alternatively, the data transmission in a downlink Cell may be scheduled using the PDCCH transmitted in the downlink Cell, and this manner is referred to as non cross carrier scheduling.
The CA technique enables a BS to transmit downlink data via multiple Cells at the same time to the same UE, and correspondingly, the UE should support feeding back Acknowledgement/Negative Acknowledgement (ACK/NACK) information for the downlink data transmitted in the multiple Cells. According to discussions about LTE-A, the ACK/NACK information for data transmission in the multiple Cells are transmitted in an uplink Cell (i.e., the uplink Pcell). LTE-A may adopt manners supporting channel selection for transmitting ACK/NACK information of up to 4 bits to support ACK/NACK information of multiple bits, and this has been adopted in LTE Time Division Duplexing (TDD) systems. When channel selection of a single antenna is taken into consideration, the number of ACK/NACKs that should be allocated is equal to the number of bits of the ACK/NACK to be fed back. That is, when the number of ACK/NACK information is M, the number of ACK/NACK channels to be allocated is M. Since each ACK/NACK channel has four available Quadrature Phase Shift Keying (QPSK) constellation points, there are 4M channels and constellation point resources, and proper channels and constellation point resources may be selected from the 4M resources for feeding back information of M bits.
According to discussions about LTE-A, channel selection in LTE-A Frequency Division Duplexing (FDD) systems only supports up to two Cells, and each Cell can be used for feeding back ACK/NACK information of 1 or 2 bits. When methods of Spatial Orthogonal Resource Transmit Diversity (SORTD) are not adopted for transmission diversity, the manner of ACK/NACK channel resource allocation includes, for the downlink Pcell, determining the ACK/NACK channel for feeding back ACK/NACK information of the Pcell by utilizing the index of a Control Channel Element (CCE) of the PDCCH in an implicit manner. For a downlink Scell, when cross carrier scheduling is not adopted or cross carrier scheduling is adopted using PDCCH of another Scell, the ACK/NACK channel for feeding back ACK/NACK information of the Scell is determined by ACK/NACK Resource Indication (ARI) in the PDCCH scheduling of the Scell. When cross carrier scheduling for a downlink Scell by utilizing the PDCCH of the Pcell, the ACK/NACK channel for feeding back ACK/NACK information of the Scell is determined by utilizing the index of a CCE of the PDCCH in an implicit manner. When a mode of transmitting a Cell is configured to be Single-Input Multiple-Output (SIMO), one ACK/NACK channel should be allocated because only one piece of ACK/NACK information for one Transmission Block (TB) of the Cell should be fed back. Accordingly, when the mode of transmitting a Cell is configured to be Multiple-Input Multiple-Output (MIMO), two ACK/NACK channels should be allocated because two pieces of ACK/NACK information for two TBs of the Cell should to be fed back. When the ACK/NACK channel is allocated in an implicit manner, the ACK/NACK channel for feeding back ACK/NACK information for one Cell is obtained from the PDCCH scheduling data transmission of the Cell. Specifically, denoting the smallest CCE index of the PDCCH as n, when only one ACK/NACK channel should be allocated, the ACK/NACK channel can be obtained by mapping of the CCE index n; when two ACK/NACK channels should be allocated, the two ACK/NACK channels can be obtained by mapping of the CCE index n and n+1.
According to discussions, backward compatibility with LTE systems is taken into consideration when mapping tables for channel selection in FDD systems are defined. Table 1 is an exemplary four-bit mapping table for channel selection when SORTD is not adopted in the related art. In Table 1, “A” denotes that ACK/NACK information received by a UE from the Scell or the Pcell is an ACK; “N” denotes that the ACK/NACK information received by the UE from the Scell or the Pcell is a NACK; “D” denotes that no data is received by the UE from the Scell or the Pcell; and “1”, “−1”, “j” and “−j” respectively denote positions of QPSK constellation points of channels selected. “ch_a”, “ch_b”, “ch_c”, “ch_d” denote four ACK/NACK channels allocated, with ch_a and ch_b being the two ACK/NACK channels for the Pcell, and ch_c and ch_d being the two ACK/NACK channels for the Scell. Specifically, when the UE only receives data from the Pcell or the ACK/NACK information received by the UE from the Scell is a NACK, the UE utilizes the first ACK/NACK channel of the Pcell (i.e., ch_a) for feeding back ACK/NACK information. When Pcell adopts MIMO, Physical Uplink Control CHannel (PUCCH) format 1b is adopted for mapping constellation points to ch_a; when Pcell adopts CC Single-Input Multiple-Output (SIMO), PUCCH format 1a is adopted for mapping constellation points to ch_a. When the UE only receives data from the Scell or the ACK/NACK information received by the UE from the Scell Pcell are NACKs, the UE uses the second ACK/NACK channel of the Scell (i.e., ch_d) for feeding back ACK/NACK information. This manner of feeding back ACK/NACK information is the same with that adopted in LTE.
TABLE 1ACK/NACK channelPcellScellch_ach_bch_cch_dA, AA, A−lA, NA, A−jN, AA, A−jN, NA, A−lA, AA, N  jA, NA, N  lN, AA, N  lN, NA, N  jA, AN, A−lA, NN, A  jN, AN, A−jN, NN, A  lA, AN, N−lA, NN, N  jN, AN, N−jN, NN, N  lA, AD, D−lA, ND, D  jN, AD, D−jN, ND, D  lD, DA, A−lD, DA, N  jD, DN, A  lD, DN, Nno transmissionD, DD, Dno transmission
In addition, according to discussions, a UE configured with only one Cell adopts SORTD for transmit diversity. More specifically, the UE is allocated with two ACK/NACK channels, and two transmitting antennas transmit the same ACK/NACK information in different channels respectively. The receiving party receives signals from the two channels and performs Maximum Ratio Combining (MRC) to obtain optimal diversity effects. The smallest CCE index of the PDCCH is denoted as n, and the two ACK/NACK channels may be obtained based on CCE index n and n+1 through implicit mapping of LTE. SORTD technique may also be adopted in the above method for feeding back ACK/NACK information of two CCs based on channel selection when transmit diversity should to be supported. The number of ACK/NACK channels used for feeding back M pieces of ACK/NACK information is 2M, where M is 2, 3 or 4. But there is no detailed solution in FDD systems and TDD systems as for how to allocate the 2M ACK/NACK channel resources.